How to define mass in the OS model?

flowwind · Post by **flowwind** » Fri Nov 17, 2006 3:22 am

Hi, Group,

I'm new using OS for my nonlinear dynamic analysis. You know, mass is a very important factor in the dynamic problem since the inertia force be included.

To my surprise, I can not find any information about the definition of mass in the OS model until I have find example5.1. However, in this example, mass is defined by way of gravity load which is calculated from column capacity. I think it is not an accurate way.

Anybody here has idea about how to do it? Any suggestions are high appreciated!

Regards

Michael Liu

pejman_opensees · Post by **pejman_opensees** » Fri Nov 17, 2006 4:16 am

Why dont you use mass command? it is straightforward!.

flowwind · Post by **flowwind** » Sat Nov 18, 2006 1:33 am

Hi Pejman, Thanks for your reply.

I have found the mass command both in user manual and some examples, but I think it is not a convenient and accurate way. As you know, when we construct a simple model, this way is feasible, such as one frame which only consists of two bays and one storey. However, when our analytical model is quite complicated, the mass is cannot calculated easily.

In some other FE software (like ANSYS), mass matrix is formed automatically if we define material density. But in the OS, we have to define the mass one by one. It is a tough work.

My question is: how to define the mass efficiently? Can we import the mass matrix formed in ANSYS to the OS model?

I would appreciate suggestions from the group... Thank you for your help!

Michael Liu

endryus · Post by **endryus** » Sun Nov 19, 2006 11:16 pm

You can use the -mass command applied on the elements.
But this option still has problems: for example, with the nonlinearBeamColumn it does not work with UniformExcitation.
I think this should be an important bug to fix. Could some developers work on it?

Andrea

flowwind · Post by **flowwind** » Mon Nov 20, 2006 12:56 am

Hi, Andrea

Thank you for your reply!

The nonlinearBeamColumn element cannot work with UniformExcitation command? I think Example 5.1.tcl can give you an illustration using these commands simultaneously.

Regards.

Michael Liu

endryus · Post by **endryus** » Mon Nov 20, 2006 1:16 am

Try to have a look to this post:

Posted: Fri Apr 22, 2005 10:38 am
Post subject: modeling cables

Andrea

pejman_opensees · Post by **pejman_opensees** » Mon Nov 20, 2006 9:27 am

As you may have noticed by now,even in complicated framed structures you use nonlinear beam-column element, otherwise you wont get a realistic model. Nonlinear beam column element has on option which is specialized for mass per unit volume exactly like what Ansys has.
hope it will work

flowwind · Post by **flowwind** » Tue Nov 21, 2006 5:06 pm

Hi, pejman,

Thanks for your reply! I have found the mass density command and it work well!

Michael Liu